The Large Hadron Collider has restarted, with protons circling the machine’s 27km tunnel for the first time since 2013. Particle beams have now travelled in both directions, inside parallel pipes, at a whisker below the speed of light. Actual collisions will not begin for at least another month, but they will take place with nearly double the energy the LHC reached during its first run. Scientists hope to glimpse a “new physics” beyond the Standard Model.

Rolf Heuer, the director-general of Cern, which operates the LHC, told engineers and scientists at the lab: “Congratulations. Thank you very much everyone… now the hard work starts”. Cern’s director for accelerators and technology, Frédérick Bordry, said: “After two years of effort, the LHC is in great shape. “But the most important step is still to come when we increase the energy of the beams to new record levels.” The beams have arrived a week or so later than originally scheduled, due to a now-resolved electrical fault.

The protons are injected at a relatively low energy to begin with. But over the coming months, engineers hope to gradually increase the beams’ energy to 13 trillion electronvolts: double what it was during the LHC’s first operating run. After 08:30 GMT, engineers began threading the proton beam through each section of the enormous circle, one-by-one, before completing multiple full turns. It was later joined by the second beam, in parallel.

The experiment teams have already detected “splashes” of particles, which occur when stray protons hit one of the shutters used to keep the beam on-track. If this happens in part of the pipe near one of the experiments, the detectors can pick up some of the debris.